In recent decades, numerous proposals have been made for providing in situ support for the face of a passage in a geological structure, such as the roof in an underground mine. A typical arrangement employs an anchor, such as an elongated roof “bolt,” that extends into a borehole formed in the face and is grouted in place. Federal regulations pertaining to underground mine safety require the placement of these bolts at frequent intervals throughout the mine passage. Consequently, ease of manufacture and use, as well as reliability, are important considerations in terms of reducing the overall installation cost to the mine owner (which, of course, directly correlates to the profitability of the mining operation).
Currently, a popular approach for roof support is the so-called “cable bolt.” This type of bolt comprises a length of flexible metal cable inserted into the borehole and grouted in place, and may be either “passive” or tensionable. In one tensionable version, the bolt includes an externally threaded tension head including opposed, longitudinally extending anti-rotation keys for engaging the sidewalls so as to prevent rotation (see, e.g., U.S. Pat. No. 3,077,809 to Harding et al.). To tension the bolt, an associated nut is advanced against an engagement structure, such as a plate, which serves to support the corresponding face in the desired fashion.
Despite the popularity of the basic tensionable approach over the years, several basic limitations remain. For one, the strata adjacent the mine passage settle or shift over time, which may cause a change in the tension originally applied during the initial installation. Likewise, the bolt over time may experience a loss in tension due to factors such as relaxation or creep. Nevertheless, most existing approaches cannot undergo re-tensioning in any reliable fashion after the initial installation.
Additionally, the current approach for installing passive cable bolts can lead to undesirable “false” tensioning and deleterious “kick back.” Specifically, the cable may continue to twist within the borehole upon the application of torque. This can lead the installer to believe that the applied torque tensions the cable bolt, when in fact it is simply causing it to twist (and thus the moniker, “false” tensioning). In some circumstances, this twisting can even cause the bolt to counter-rotate, or “kick back,” upon release of the accumulated energy, which is undesirable for obvious reasons.
Accordingly, a need exists for an improved bolting apparatus that overcomes the foregoing limitations of the prior art. Specifically, the bolt should be easy and inexpensive to manufacture and install, without the need for bulky castings that would extend below the roof line. In one particularly preferred embodiment, the bolt would be also be tensionable to compress and provide secure, reliable support for the adjacent strata once installed, as well as re-tensionable at a later time should the need arise.